The present invention relates to aqueous coating compositions and to their use for coating plastic substrates. More specifically, the present invention relates to aqueous coating compositions that contain chlorinated polyolefin dispersions and polyurethane dispersions. These coating compositions produce dried films which adhere well to plastic substrates while maintaining good application properties over a broad range of application conditions.
Plastic substrates are widely used in automotive parts, container caps and containers for cosmetics, household appliances and other applications. Organic coating compositions are often applied to such plastic substrates for decorative and protective purposes. An ongoing problem with respect to such use of these substrates has been the difficulty in achieving adequate adhesion of organic films to the plastic, especially after the film has been aged or exposed to environmental stress.
Complicating the problem is the fact that there are many different types of plastics used commercially, each having unique surface properties. It would be advantageous if one product could be used to coat a variety of substrates that might vary widely in properties such as surface tension, surface roughness, chemical composition, and hardness.
In automotive applications such as the formulation of automotive refinish compositions, there are other requirements besides adhesion that must be met; for example, there are requirements for compatibility with other coating compositions which can be applied over the coating in question; for stability upon long-term storage or upon exposure to shear; for room-temperature cure; for a high degree of resistance of the dried film to solvents, fuels, acids and other chemicals; for a high degree of resistance of the dried film to humidity; for excellent appearance; and for low cost.
A wide range of approaches has been proposed to overcome the adhesion problem. Perhaps the most viable has been the application of a primer composition that contains an aqueous chlorinated polyolefin dispersion and an aqueous acrylic dispersion. While this is a low-cost material, it provides only fair adhesion, showing weaknesses especially after exposure to humidity. Even with this material, it is still difficult to economically produce dried films with excellent adhesion, application properties and physical properties.